Nuclear and Emerging Technologies for Space, American Nuclear Society Topical Meeting Richland, WA, February 25 – February 28, 2019, available online at http://anstd.ans.org/ NUCLEAR HEAT SOURCE CONSIDERATIONS FOR AN ICY MOON EXPLORATION SUBSURFACE PROBE Daniel P. Kramer 1, Christofer E. Whiting 1, Chadwick D. Barklay 1, and Richard M. Ambrosi 2 1University of Dayton Research Institute, 300 College Park, Dayton, Ohio, 45469 937-229-1038; [email protected] 2University of Leicester, Space Research Centre, University Road, Leicester, UK, LE1 7RH RTG powered spacecraft have enabled the TABLE I. Several icy moons within the Jovian and identification of several icy moons within the solar system Saturnian and Ice Giant systems. which may contain sub-surface oceans of water below a Planet Moon Main thick ice cap. Inserting a probe into one of these oceans Exploratory may assist in determining whether Earth is the only place Spacecraft in the solar system where life forms have existed attempting to answer the age long question of whether we Jupiter Europa Galileo are alone in the universe. It is reported that the ice shell Ganymede Voyager of some of the most interesting icy moons may be as much Callisto as tens of kilometers thick. One concept discussed in the literature is to employ plutonium-238 as a heat source Saturn Enceladus Cassini within a probe to melt through the moon’s ice shell to the Voyager liquid ocean. This would then allow the investigation of the ocean environment. Uranus Miranda Voyager Umbriel This paper discusses considerations for helping to identify potential radioisotope heat source for an icy Neptune Triton Voyager moon probe, such as: thermal power output, half-life, future availability, etc. Additionally, a first-order analysis infers that two radioisotopes (curium-244 and There is increasing interest in the development of uranium-232) exhibit a number of the characteristics concepts for the possible future exploration of an ocean likely required for a future ocean probe to one of the icy on one of these moons. This appears to be a very moons. The analysis suggests that compared to the mass complex problem consisting of various challenges, which of plutonium-238 required, 244 Cm could require 75% less may include: transit time; entry, decent, and landing mass and 232 U would require 88% less mass, while still (EDL); radiation levels from both the selected yielding a similar thermal output. In addition to these radioisotope and from the mission environment; options, consideration is given to polonium radioisotopes “melting/cutting” through the ice cap; and (Po-208 in particular) as a potential alternative . The communications. authors highly recognize that the selection of any new Over the years, the planetary exploration community radioisotope heat source material will require extensive; has developed a number of different mission concepts radiological considerations, realistic evaluation of employing various spacecraft scenarios both in the U.S. obtainability, cost factors, and launch safety (Jupiter Icy Moons Orbiter – JIMO and the Europa considerations. Clipper) and in Europe (Jupiter Icy Moons Explorer – I. INTRODUCTION JUICE). Several exploratory spacecraft, such as Galileo The JIMO mission development contract was (Jupiter) and Cassini (Saturn), both powered by awarded by NASA to Northrop Grumman in 2004. The plutonium-238 fueled General Purpose Heat Source - initial mission concept included an orbiter which would Radioisotope Thermoelectric Generators (GPHS-RTG), arrive at the Jovian system in 2021 to study Callisto, have helped to identify several moons in the solar system Ganymede, and Europa, which included a lander to which may have oceans below their observable surfaces. investigate the surface of Europa. This program was These moons are often called “icy moons” and are of cancelled, and currently NASA is funding the Jet considerable scientific interest; since it is believed that Propulsion Laboratory (JPL) to further develop the their oceans may contain the right conditions to have Europa Clipper mission concept. This mission concept biologically supported the formation of life. Table 1 lists would position a spacecraft in an elliptical orbit around several icy moons within the Jovian and Saturnian Jupiter with a number of close flybys of Europa in the systems. mid- to late 2020s timeframe. The JUICE orbiter mission is an approved ESA availability, cost, etc.) of the radioisotopes discussed will mission with the spacecraft being developed by Airbus need to be critically addressed in future endeavors. Defence and Space. The JUICE mission currently centers II.A. Radioisotope selection considerations on studying several Jovian moons including Callisto, Ganymede, and Europa. Orbital insertion around Jupiter A Europa surface mission with a probe capable of could be in the ~2030 timeframe. If both Europa Clipper reaching and investigating the sub-surface ocean would and JUICE are launched, the two mission profiles would be one of considerable duration. Assuming a future + complement each other, and would provide a more spacecraft takes ~6 years to get to Europa, 2 or 3 years comprehensive understanding of several of Jupiter’s icy for the probe to melt/cut through the kilometers thick ice moons particularly of Europa. shell, and a 2 or 3 year ocean exploration. For this paper it is assumed probe landing on Europa occurs at 10 years In addition, a number of recent studies have been after launch with total mission duration of 15 years. For carried out exploring what missions could be feasible the RTG being employed on the upcoming Mars2020 further out in the solar system. Notably the recent NASA mission, a 3 year pre-launch time duration was also ESA collaboration on missions to the ice giants explores 4 included in the mission plan. the possibility of using probes and landers to complement orbiting spacecraft or planetary flybys. 1 Examining a standard Chart of the Nuclides it becomes quickly apparent that there are literally hundreds Execution of these concept missions would be very of radioisotopes that might meet these general advantageous in the development of future missions to the 5 requirments. However, in general there are several icy moons. These missions could also assist in a future property characteristics which can be used to “cull” the icy moons mission by obtaining a more accurate list including; a) present and/or future attainability, b) measurement of the thickness of the ice cap on the activity sufficient to obtain the required magnitude of targeted mission locations. This would allow for a more thermal energy, c) a half-life long enough to provide the informed development of an icy moons probe that could required thermal energy for the duration of a selected melt/cut through the kilometers of ice to reach the liquid mission, and d) potential for processing into a non- ocean. metallic form such as an oxide which would likely assist One innovative probe concept employs a “CHIRPS” in a future launch safety analysis. Other possible (Cryo-Hydro Integrated Robotic Penetrator System) that selection criteria would be to identify radioisotopes that would employ four GPHS modules and several are primarily alpha or beta emitters with lower energy Radioisotope Heater Units (RHU).2 Each GPHS (as gammas. These characteristics are why plutonium-238 employed on the Galileo and Cassini spacecraft) contains has been extensively employed over the last several 238 four ~150 gm PuO 2 ceramic pellets that each produce decades as the radioisotope fuel in U.S. RTGs. However, ~62.5W th yielding ~1kW th in total for a notional CHIRPS. if the existing U.S. plutonium-238 based RPS 238 Each RHU contains ~2.7 gms of PuO 2 which yields infrastructure was not the primary consideration, it may ~1W th . Plutonium-238 is an alpha emitter with a thermal likely not be the preferred radioisotope for a Europa probe output of ~0.5W th /g with a half-life of ~88 years. It is mission based mainly on its relatively long half-live and envisioned that the four GPHS modules would be relatively low thermal output. The question about the employed to melt/cut through the ice shell and the RHUs ability to fabricate and ultimately launch a particular could be employed as mini-RTGs to produce enough radioisotope in relation to general launch safety electrical power for the probe. Ultimately it was considerations is also being left to future investigations. determined that a thermal output of ~1kW would likely th Table 2 is a selected list compiled from several be insufficient for a full mission profile. Recent work sources of radioisotopes which exhibit several of the suggests that the thermal load required to “melt” through aforementioned criteria that could possibly enhance a kilometers of Europa’s ice shell could be on the order of 6,7,8 Europa probe melting mission. These radioisotopes ~7.5 kW , equivalent to the thermal output of 28 or 30 th are mainly alpha emitters, but in many cases they and GPHS modules. 3 This corresponds to ~16.8 to 18kg of 238 their various decay chains, also emit either betas and/or PuO 2. gammas. Shown in the table as a comparison are the heat II. OTHER NUCLEAR MATERIALS OPTIONS source characteristics of polonium-210 which was the original U.S. RTG heat source material, plutonium-238 It appears that to melt through Europa’s ice sheet a probe may require ~7.5 kW requiring a significant which is employed as the heat source in current U.S. th RTGs, and americium-241 which has been selected as the quantity of plutonium-238. This present discussion is a heat source material for future ESA RTGs.9 first-order effort at identifying some possible radioisotopes that could maybe supply the required thermal load. Potential shortcomings (high activities, 2 TABLE II. Selected radioisotopes which exhibit one or It should be noted that Figure 1 is not a perfect more of the discussed criteria characteristics for a Europa reflection of the heat output from a given isotope.